Re: [PATCH 03/16] pack-objects: use a faster hash table

[Jeff King <peff@peff.net>]

On Tue, Jun 25, 2013 at 10:14:17PM -0400, Jeff King wrote:

> So I suspect two things (but as I said, haven't verified):
> 
>   1. You could speed up pack-objects just by keeping the table half full
>      rather than 3/4 full.

I wasn't able to show any measurable speedup with this. I tried to make
as specific a measurement as I could, by adding a "counting only" option
like this:

diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index fc12df8..a0438d0 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -2452,6 +2452,7 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 	const char *rp_av[6];
 	int rp_ac = 0;
 	int rev_list_unpacked = 0, rev_list_all = 0, rev_list_reflog = 0;
+	int counting_only = 0;
 	struct option pack_objects_options[] = {
 		OPT_SET_INT('q', "quiet", &progress,
 			    N_("do not show progress meter"), 0),
@@ -2515,6 +2516,8 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 			    N_("pack compression level")),
 		OPT_SET_INT(0, "keep-true-parents", &grafts_replace_parents,
 			    N_("do not hide commits by grafts"), 0),
+		OPT_BOOL(0, "counting-only", &counting_only,
+			 N_("exit after counting objects phase")),
 		OPT_END(),
 	};
 
@@ -2600,6 +2603,8 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 		for_each_ref(add_ref_tag, NULL);
 	stop_progress(&progress_state);
 
+	if (counting_only)
+		return 0;
 	if (non_empty && !nr_result)
 		return 0;
 	if (nr_result)

and even doing the whole object traversal ahead of time to just focus on
the object-entry hash, like this:

  git rev-list --objects --all >objects.out
  time git pack-objects --counting-only --stdout <objects.out

Tweaking the hash size didn't have any effect, but using Vicent's khash
patch actually made it about 5% slower. So I wonder if I'm even
measuring the right thing. Vicent, how did you get the timings you
showed in the commit message?

-Peff